Automotive lighting systems are increasingly making use of semiconductor light sources, such as light emitting diodes (LEDs), due to their reliability, power efficiency and the reduced amount of waste heat they produce, compared to incandescent light sources. With improvements in semiconductor devices, it has recently become possible to construct high output lighting systems, such as vehicle headlamp systems, using LED light sources.
However, while semiconductor light sources do offer advantages over other light sources, such as incandescent or gas discharge sources, they also have some weaknesses. In particular, LEDs are susceptible to over-voltages, wherein too much voltage is applied to their semiconductor junctions, resulting in too much current flowing through the semiconductor junctions, damaging the LED and shortening its life. Also, if too little current is supplied, LEDs produce less light (fewer lumens) and the lighting system may not output sufficient lumens to meet safety and/or regulatory requirements.
As automotive electrical systems typically experience relatively wide voltage swings and as automotive lighting systems typically must operate over wide temperature ranges and conditions, it has been difficult to provide appropriate electrical power to semiconductor light sources at a reasonable cost.
In addition to controlling the electrical power supplied to the LEDs, it can also be desirable to turn some LEDs on and some off. For example, a headlamp may have LEDs which are only illuminated when the headlamp is forming a high beam pattern. In prior art systems, a power supply would be provided for each set or group of LEDs to be separately illuminated and, while such a design could provide the desired flexibility, it was also quite expensive.
Also, as the characteristics of the semiconductor junctions in each LED vary, it is difficult to connect LEDs in parallel to the power supply as the parallel connected LED with the lowest junction resistance would receive too much current while the parallel connected LED with the highest junction resistance would receive too little current. Thus parallel connected semiconductor lighting systems are generally avoided. However, series connected semiconductor light sources also suffer from disadvantages in that the failure of a single semiconductor light source (which generally fail as open circuits) results in the failure of the entire series connected string of semiconductor light sources. Further, such series connected power supplies have been unable to provide for the dimming of some LED light sources in a lighting system. Any dimming of an LED in the series would result in every other LED also being dimmed.
It is desired to have a power supply for semiconductor-based automotive lighting systems, particularly high output lighting systems such as headlight systems, which is not subject to these problems.